The APP extension is used by the Apple Macintosh OS X operating system to identify executable applications. These can simply be started by double-clicking. They resemble the exe files under Microsoft Windows.
The file is a container that contains the actual executable files. This file is displayed under Mac OS X as a single executable file. In Microsoft Windows, however, as a folder with the suffix app, which contains the files of the application.
These files may be infected with malware and should therefore be tested with an antivirus program.
Most often resolving problems with opening APP files is very simple. Just install an appropriate program that supports such files. All of the listed programs support APP files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of APP files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate APP files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the Mac OS X applications is not on the list, select "Browse" option in order to manually locate the directory where Mac OS X has been installed.
Sometimes problems with opening APP files may not lie with the application itself, but can arise due to other causes, such as: